A biaxially oriented polyethylene-2,6-naphthalenedicarboxylate film is excellent in mechanical properties, thermal properties and electric properties, and is hence studied for use in various fields. It has been put to practical use in the fields of a magnetic recording medium and electric insulation.
As one of uses of the above film, it is used in the field, for example, of photographic photosensitive materials. These photosensitive materials are largely classified into films used in the form of a sheet such as an X-ray film, a film as a printing plate and a cut film and films used in the form of a roll typified by a color or black-and-white negative film having a width of 35 mm or less. This negative film is encased in a film cartridge and the cartridge is placed in the holder of a general camera when used for taking pictures.
A biaxially oriented polyester film formed of polyethylene terephthalate is mainly used as a base for films in the form of a sheet. Further, a film formed of a cellulose-based polymer typified by triacetyl cellulose (to be sometimes referred to as TAC hereinafter) is used as a base for films in the form of a roll.
A TAC film has its characteristic features in freedom from optical anisotropy, high transparency and the property of removal of curling after developing treatment, and in particular, the excellence in the property of removal of curling is said to be a feature that does not belong to a film of any other material. Since, however, the production of the TAC film uses an organic solvent, it is necessary to take care to recycle the solvent completely for preventing the environmental pollution. In particular, the issue of environmental destruction is attracting attention in recent years, and there is a growing tendency to decrease or avoid the use of an organic solvent which may cause an environmental pollution.
On the other hand, a polyethylene terephthalate film is a film that can be formed by a melt extrusion method without any organic solvent, and a polyethylene terephthalate film is partly started to be used as a base film for a photosensitive film for photography.
However, a polyethylene terephthalate film has a problem in that it undergoes curling (curling tendency) and that it is difficult to remove the curling. That is, a base film for a photosensitive material for photography is once taken up and stored in the form of a roll, and thereafter the film is taken out of the storage, cut to a proper size and used in the form of a flat sheet such as an X-ray film, a printing plate or a cut film. In the case of a rolled film, after taking pictures, development and fixing, a negative film encased in a film cartridge in the form of a roll is cut to a predetermined length and inserted into a film album, and the film is stored in the form of sheets. However, a polyethylene terephthalate film has a problem in that it retains the curling which has occurred when the film has been taken in the form of a roll, and it curls after cut in the form of sheets.
For overcoming the curling by easing the removal of the curling which has occurred, or by decreasing the curling tendency to prevent the curling, the following proposals have been made.
Japanese Laid-open Patent Publication No. 16,783/1975 discloses a biaxially oriented laminated polyester film formed of two polyester layers, in which the intrinsic viscosity of each polyester layer is in the range of 0.35 to 1.0 but the difference between the intrinsic viscosity of one polyester layer and that of the other polyester layer is in the range of 0.02 to 0.5.
The above laminated polyester film seeks to overcome the curling by causing a curling based on a shrink stress difference between the front surface layer and the opposite surface layer and taking up the film in the direction opposite to the curling direction.
Japanese Laid-open Patent Publication No. 81,325/1975 discloses a photographic film whose base film is a biaxially oriented polyethylene2,6-naphthalenedicarboxylate film in which the ratio of Young's modulus in longitudinal direction to Young's modulus in transverse direction is in the range of 0.9 to 1.1, the saturated shrinkage percentage or saturated expansion coefficient at 180.degree. C. is 0.9% or less, the difference between the saturated shrinkage percentage or saturated expansion coefficient in longitudinal direction and that in transverse direction at 200.degree. C. or lower is 0.4% or less and the haze is 4.5% or less.
Japanese Laid-open Patent Publication No. 95,374/1975 discloses a process for producing a polyester film, characterized by biaxial orienting, heat setting and aging under heat in the temperature range of 40.degree. C. to 130.degree. C. In its Example, there is disclosed a polyethylene-2,6-naphthalenedicarboxylate film having 12 .mu.m in thickness, obtained by the biaxial orienting at 4.3 times in the longitudinal direction and 3.5 times in the transverse direction, the heat setting at 200.degree. C. and the aging in the temperature range of 40.degree. to 130.degree. C. for 24 hours.
Japanese Laid-open Patent Publication No. 109,715/1975 discloses a photographic film of which at least the base film is formed of a film which is formed of a polyester having an intrinsic viscosity (35.degree. C., in o-chlorophenol) of at least 0.40 and containing ethylene-2,6-naphthalenediearboxylate in an amount of at least 90 mol % of whole constituent units and which has a haze of less than 5% and is biaxially oriented and heat set.
U.S. Pat. No. 4,141,735 discloses a method of decreasing the core set curling property of a film without substantially deforming or shrinking the film by heat-treating a self-supporting film having a thickness of approximately 5 to 50 mm and being formed of a thermoplastic polymer having a Tg, measured by DSC at a heating rate of 20.degree. K./minute, of higher than about 60.degree. C. This method is carried out by maintaining the film at a temperature between 30.degree. .C and the Tg of the above polymer at a relative humidity of 100% or less for about 0.1 to about 1,500 hours until the core set eurling property decreases by at least 15%. This decrease in the core set curling property is measured by comparing a numerical change at an ANSI curl unit when a heat-treated film has been core-set on a core having an outer diameter of 3" at 49.degree. C. at 50% RH for 24 hours with a numerical change at an ANSi curl unit when the corresponding film not heat-treated has been core set in the same way.
In Example 10 of the above U.S. Patent, Table 7 shows temperatures for heat-treating a poly(ethylene-2,6-naphthalenediearboxylate) film having a Tg of 198.degree. C. and net ANSI curl values thereof in core set curling properties, and it is shown that when the treatment temperatures were 60.degree. C., 71.degree. C., 100.degree. C., 120.degree. C., 149.degree. C. and 180.degree. C., the net ANSI curl values were 18, 16, 13, 16, 20 and 25, respectively.
Japanese Patent Publication No. 53,745/1981 discloses a photographic film obtained by providing, as a base film, a laminated polyester film which is formed of two polyester layers respectively having intrinsic viscosities in the range of 0.35 to 1.0 and having an intrinsic viscosity difference between the layers in the range of 0.02 to 1.00 and which has curling attained by the stretching heat setting, and applying a photosensitive layer to a layer side of the base film, which side has a lower intrinsic viscosity, so that the curling of the base film is offset by the shrinkage of the photosensitive layer.
Japanese Laid-open Patent Publication No. 244,446/1989 discloses a photographic photosensitive material having a polyester base film having a haze of 3% or less and a water content of 0.5% by weight or more and at least one photosensitive layer. The feature of this photographic photosensitive material is that the base film has a water content of at least 0.5% by weight, and for obtaining this water content, an aromatic dicarboxylic acid component having metal sulfonate is copolymerized.